Vehicle Side 4-Way Trailer Connector Plug-in Circuit Tester Lights Light Up when Connected

Question:

When I plugged in the 4 prong trailer tester, all three green lights flashed. What does that indicate?

asked by: Dan F

Expert Reply:

If the lights on a 4-Way trailer connector tester such as # I26 light up when connected that means that those circuits are receiving power. It doesn't take much power to light up these small LED lights so even residual current could set them off. This is typically more likely on aftermarket installed trailer wiring harnesses that have a connection to the battery to power the trailer lights.

It could also indicate a short circuit if the lights stay on all the time. The likely cause is a dirty or corroded trailer connector or damaged wiring with copper strands touching bare metal. I have included some links to the right that should help troubleshoot.

expert reply by: Michael H